1. Field of the Invention
This invention relates to a process control system, and more particularly to a centralized supervisory process control system which uses digital controllers for distributed processing.
2. Description of the Prior Art
Referring to FIG. 1 schematically showing a typical centralized process control system, a plurality of digital controllers 1 for distributed processing are connected to transmission lines 2 which lead to a centralized supervisory computer (to be referred to as "supervisory computer", hereinafter) 3. Each controller 1 for distributed processing carries out process control of single-loop or multi-loop type. The function of the supervisory computer 3 includes
(i) the centralized supervision of temporary loop parameters, i.e. measured values of various variables, the magnitudes of manipulated variables, and the like of each control loop under its supervision (to be collectively referred to as the "temporary parameters", hereinafter), by collecting them through the transmission lines 2 and displaying them; PA1 (ii) the centralized supervision of control program parameters, i.e., various set points, the proportional gain (or proportional band PB) and the integral action rate (or integral time I) and the derivative action time constant (or derivative time D) of the proportional plus integral plus derivative (P.I.D.) control action, upper/lower limits for alarming, and the like of each control loop under its supervision (to be collectively referred to as the "control parameters", hereinafter), by collecting them through the transmission lines 2 and displaying them; and PA1 (iii) centralized supervision and control, such as modification of the above-mentioned control parameters from the supervisory computer 3 through the transmission lines 2. PA1 (a) The controller tends to have as many software operating modules mounted thereon as possible provided that they have a finite probability of actual use in the field, so that the number of operating modules increases with the expansion of its application, resulting in an increased memory capacity. Thus, the controller having a wide application has an increased memory capacity, and hence its mean time between faults (MTBF) is reduced and it becomes costly and uneconomical. PA1 (b) Although it is true that the number of plants requiring a large number of operating modules to form complicated control loops and to carry out complicated control operations is increasing, the majority of the controllers actually installed are still used with simple loop formation and comparatively simple control operations. Thus, for controllers to be used with simple control loops, provision of those software modules which are not used is uneconomical. PA1 (c) In the above digital controller of the prior art, the software is complicated. Besides, the operation of the ROM writer for making the software wiring requires certain degree of experience. PA1 (d) Before replacing a faulted controller with a spare controller, it is necessary to write the software wiring on a ROM by a ROM writer and mount the ROM on the spare controller. Alternatively, the software wiring may be loaded from a keyboard or a cassette. In any case, due to the need of the software wiring, very quick replacement with the spare controller is hard to achieve even in emergency. PA1 (e) When a fault occurs, the following difficulties are encountered in the conventional controller.
FIG. 2 shows a block diagram of a typical digital controller 1 for distributed processing of the prior art. Analog input signals IA1 through IAn from a process being controlled are applied to a multiplexer 101, so that they are successively converted into digital signals by an analog/digital converter 102. The digital signals from the analog/digital converter 102 are delivered to a bus 103 and stored in a memory 113. A display with keyboard switches 115 mounted on the front panel and sidewall of the controller 1 is connected the bus 103 through a display interface circuit 114. Digital input signals ID1 through IDn from the process are stored in the memory 113 through digital input interface circuit 105 and the bus 103.
Various data carried by the stored input signals are processed by the control program of the controller 1 at the central processing unit (CPU) 112. The processed signals are applied either to an output holder 108 through a digital/analog converter 106 and a demultiplexer 107, or to another output holder 110 through a digital output interface circuit 109. The output holders 108 and 110 have dual functions; namely, to hold the above output signals for a period corresponding to the sampling period of the digital system of the controller 1 and, in the case of any fault in the controller 1, to hold the levels of above output signals immediately before the fault occurrence. A transmission interface circuit 104 connected to the bus 103 acts to transmit data from the controller 1 toward the outside circuit and to collect data from the outside circuit. Preferably, the transmission interface circuit 104 is connected to the transmission line 2 of FIG. 1.
The software of the digital controller 1 for distributed processing is often in the form of a program mounted on a read only memory (ROM). More particularly, from the standpoint of standardization and interchangeability, subroutines for those unit mathematical operations and unit control operations which are expected to be frequently used are written on a ROM as software modules (to be referred to as the "operating modules", hereinafter), as shown by the operating modules or subroutines 203a through 203k on a ROM 203 of FIG. 3A. Preferably, the ROM 203 is made as a part of the memory 113 of FIG. 2, and it may be in the form of an eraseable programmable ROM (EPROM) or a mask ROM. A desired control program of software is written by connecting only those operating modules which are necessary for actual control by a software wiring 202 to be described hereinafter.
The software wiring 202 of the operating modules will be described by referring to an example of cascade control of FIG. 3B. In the control system of FIG. 3B, to control the temperature in the furnace 21, the fuel flow through a fuel pipe 22 is detected by a flow meter 23, and the flow rate data signal is delivered to the controller 1. Based on temperature data signal from a thermometer 24 monitoring the temperature in the furnace 21 and the above flow rate data signal, the controller 1 manipulates the opening of a fuel valve 25, so as to control the combustion at a burner 26 and accordingly the temperature in the furnace 21.
Referring to FIG. 3C showing a block diagram of the software for effecting the above cascade control, the temperature data signal is represented by an analog input signal IA1 applied at an input point 201. To make correction for the non-linearity of the detector, or the thermometer 24, the analog input signal IA1 is processed by a linearizer module 203e. The corrected signal is applied to a PID module 203f. The output signal from this PID module 203f is used as a set point for another PID module 203f. Similarly, the operating modules in the block diagram of FIG. 3C are connected. It is noted that the connection among the operating modules of FIG. 3C is made not by hardware wiring but by software wiring.
In FIG. 3C, such software wiring 202 is enclosed by the broken lines. In practice, the entire software wiring 202 is preferably written on a ROM by a ROM writer in the field so as to meet the needs of actual processes to be controlled. The ROM thus written is mounted on the controller 1 to make it ready for control operation. Alternatively, the software wiring may be loaded on a non-volatile memory of the controller 1 from a keyboard or a cassette recorder before starting the control operation.
Accordingly, as far as the hardware is concerned, the above-described controller 1 for distributed processing intrinsically lacks individuality, and its individuality is given in the field by providing the software wiring when it is applied to the actual plant. Thus, the controllers which lack hardware individuality can be universally applied to a variety of plants by giving required individuality through software wiring in the field. However, the conventional control system using such controllers has the following shortcomings.
To avoid this disadvantage, one may think of provision of two kinds of controllers, one with simple formation and one with highly complicated formation, but this is against the merit of standardization and inter-changeability.
As OUTPUT HOLD, in case of a fault, the manipulated variable to be applied to the process being controlled is held at a level immediately before the occurrence of the fault, and such level of the manipulated variable is kept until the recovery from the fault. However, this method cannot respond to any change in the process after the occurrence of the fault, so that it is very dangerous to use this method as a backup for an extended period of time. PA2 As HARD MANUAL, a circuit for manual control of the manipulated variable applicable to the process being controlled is separately prepared by using only those instruments which have a comparatively low rate of fault, such as the power source, potentiometers, and the like, so that upon detection of the occurrence of a fault, the output from the controller is automatically switched to that of the above circuit for manual control. With the HARD MANUAL, continuous supervision by operating persons and manual control operation are necessary until the recovery of the normal operation either by the repairing of the faulted controller or by replacement of the faulted controller with the spare controller. However, it is practically impossible to effect manual backup operation in case of a control system including a complicated loop formation.
In short, the controller of the prior art does not have any satisfactory backup in the case of fault.